Did it occur to you that you were just teaching them to disrespect you? If there was no other way to get the parts there, fair enough; if the profit was such that eating the parking was not a problem, fair enough.
But the previous posters are portraying a situation where someone could have (in fact promised to) rectify a difficult situation. Also that the costs ate up the profit, if they would have just swallowed it.
That's not working smart. I invested in myself to become skilled, not somebody's doormat. I don't work for free, I work for a FAIR wage. I will bend over backwards for the customer. I will NOT bend over forwards.